1. Field of the Invention
The present invention is related to a control of discharge energy of an electric discharge machine for supplying processing energy between an electrode and a workpiece provided in a processing fluid.
2. Description of the Related Art
As a conventional electric discharge machine, FIG. 5 represents the electric discharge machine disclosed in Japanese Laid-open Patent Application No. 57-138530. In FIG. 5, the electric discharge machine is equipped with an electrode, a DC power supply 24 having a variable voltage applied to a workpiece 2, and another DC power supply 26 having a constant voltage value V1. The output of the DC power supply 24 is switching-controlled by a sub-switching element 28, and the output of the DC power supply 26 is switching-controlled by a main switching element 30. It should be noted that reverse-current blocking diodes 46 and 48 are series-inserted into the respective power supplies 24 and 26.
This electric discharge machine is constituted by a delay-differential circuit 34 connected to an output of an oscillator 32 for periodically producing a pulse signal; a one-shot multivibrator 38 operated in response to a rising edge of an AND-gate output from an AND circuit 36; an amplifier 42 for amplifying a current flowing when the electric discharge occurs by either the sub-switching element 28 or the main switching element 30 by detecting a voltage produced across a shunt resistor 40; and a judging apparatus 44 for producing a high-output when the output of this amplifier 42 is located within a predetermined range, and a low-output when the output of this amplifier 42 is located outside this range.
Referring now to FIG. 5 and FIGS. 6A to 6G, operation of the electric discharge machine with the above-described arrangement will be described. First, when the power supply is turned ON, a rectangular pulse signal S1 (see FIG. 6B) is generated from the oscillator 32, and then the sub-transistor 28 is turned ON to thereby apply a voltage V2 (see FIG. 6A) to the electrode 3 and the workpiece 2. The pulse signal S1 of the oscillator 32 is processed by the delay/differential circuit 34 to produce an output signal S2 (see FIG. 6C) by a time delay Td.
On the other hand, the electric discharge occurs between the electrode 3 and the workpiece 2, so that the current flows, and the voltage across both terminals of the shunt resistor 40 is amplified. When the amplified voltage reaches a predetermined voltage value, the judging apparatus 44 outputs a high-signal S3 (see FIG. 6D). This signal S3 and the output signal S2 of the delay/differential circuit 34 are AND-gated by the AND circuit 36 to thereby produce a signal S4 (see FIG. 6E). A vibrator 38 is triggered in response to this output signal S4, and the main switching element 30 is driven in response to an output signal S5 of the vibrator 38, so that the electric discharge between the electrode 3 and the workpiece 2 is continued.
However, the conventional electric discharge machine is arranged by the above-described manner. Even when the electric discharge conditions are deteriorated, the occurrence of the electric discharge is judged by the judging apparatus 44 in such a manner that the voltage produced by amplifying the voltage across the both terminals of the resistor 40 is located in the preselected value range. Under the deteriorated condition, there are a so-called "immediate electric discharge" in which the voltage between the electrode 3 and the workpiece 2 does not reach V2, but immediately becomes Vg, and a so-called "semi-immediate electric discharge" in which after the voltage between the electrode 3 and the workpiece 2 reaches V2, and this voltage immediately becomes Vg. As a consequence, there is such a problem that even when the immediate electric discharge and the semi-immediate electric discharge happen to occur, the electric discharge machine is operated similar to the normal electric discharge operation.